Apparatus for Maintaining a Urea Solution in a Liquid State for Treatment of Diesel Exhaust

ABSTRACT

A system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures comprising a reservoir tank module disposed in a solution storage tank. Solution in the storage tank is heated partially by passage of heat through the walls of the reservoir tank module. Additional heat is derived from waste heat in engine exhaust gas and is added to the system either by passing a portion of the exhaust gas stream directly onto or through the solution or by passing air heated by the exhaust gas stream directly onto or through the solution. Alternatively, the hot gas may be impinged onto an outer surface of the storage tank.

TECHNICAL FIELD

The present invention relates to emissions control in compression-ignited internal combustion engines; more particularly, to systems for injecting urea into diesel exhaust to scavenge nitrogen oxides and rejuvenating a diesel particulate filter; and most particularly, to a system for heating and liquefying a storage tank solution of urea at normally sub-freezing urea-solution temperatures.

BACKGROUND OF THE INVENTION

To scavenge oxides of nitrogen (NOx) from the exhaust of compression-ignited (CI) engines, and especially diesel engines, urea injection systems are commonly in use in the prior art. An aqueous urea solution is injected into the hot exhaust pipe, where urea is hydrolyzed into ammonia ahead of a selective catalytic reduction (SCR) converter. Ammonia reacts with NOx trapped on the catalyst face to form N₂, CO₂, and H₂O, thereby lowering the level of noxious emissions in the exhaust.

A serious problem in the prior art is that at temperatures below about −11° C., the urea solution can freeze. Thus, a thermal heating system and method are required to thaw the solid solution into a liquid solution (or to keep the solution from freezing) to permit a pump to draw solution for delivery into an engine emissions abatement system.

A typical prior art urea supply system comprises a relatively small reservoir tank module from which liquid urea solution is dispensed into a diesel engine exhaust system, and a larger storage tank in which the tank module is immersed. The tank module contains a resistance heater that can liquefy suitable quantities of solution in a short time upon engine start-up under cold conditions, as is required to meet government air pollution standards. Solution in the surrounding storage tank then is heated by transfer of heat through the walls of the heated reservoir tank module.

It is an important operating requirement that the storage tank be able to re-supply the reservoir tank module within a short time after starting of the engine. In prior art systems when the solution in the storage tank is frozen, meeting this requirement can be difficult because of limited heat flow through the walls of the reservoir tank module, which typically is formed of a plastic polymer having relatively low thermal conductivity.

U.S. Pat. No. 6,387,336 discloses generally that an electric heating system and/or a heating device that uses waste heat of the engine coolant and/or the exhaust gas can be used to heat the frozen urea.

Published US Patent Application No. US 2008/0092531 discloses use of hot engine coolant to heat the frozen urea. The specification describes a separate coolant conduit embedded in the urea tank for heating the tank, and a double pipe construction surrounding the urea feed pipe for heating the urea flowing through the feed pipe. The inside of the double pipe arrangement provides a flow path for the urea while a jacketing-surrounding outer pipe carries the heated coolant, which may flow counter to the direction of flow of the urea. Also disclosed is use of heat from a proximate exhaust pipe.

What is needed in the art is an improved storage system for urea solution wherein frozen urea solution may be liquefied at a rate sufficient to maintain replenishment of the reservoir tank module.

It is a principal object of the present invention to provide a reliable flow of liquid urea solution at ambient temperatures below the freezing point of the solution.

SUMMARY OF THE INVENTION

Briefly described, a system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures comprises a reservoir tank module disposed in a storage tank. The reservoir tank module preferably includes a level sensing apparatus, inlet and outlet ports for supplying and withdrawing urea solution, and at least one heating element. The walls of the reservoir tank module are preferably immersed in urea solution contained in the storage tank, which solution is partially heated by passage of heat through the walls of the reservoir tank module.

In accordance with the present invention, additional heat for melting frozen urea solution is derived from waste heat in engine exhaust gas and is added to the system either by passing a portion of the exhaust gas stream directly through the solution or by passing air heated by the exhaust gas stream directly through the solution. Alternatively, the hot gas may be impinged onto an outer surface of the storage tank.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an elevational schematic view of a prior art system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures;

FIG. 2 is a schematic drawing of a first embodiment in accordance with the present invention of a system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures, showing passage of a portion of the engine exhaust gas stream diverted through the storage tank; and

FIG. 3 is a schematic drawing of a second embodiment in accordance with the present invention of a system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures, showing air heated by the engine exhaust gas stream being diverted through the storage tank.

The exemplifications set out herein illustrate currently-preferred embodiments of the present invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a portion of an exemplary prior art system 10 for supplying a solution of urea to a diesel exhaust emissions abatement system 12 for a diesel engine 14 comprises a reservoir tank module 16 disposed within a storage tank 18 for urea solution 20. Solution 20 enters tank module 16 via an inlet 22 and is dispensed via an outlet 24. A heater 26 is disposed within tank module 16 for liquefying solution 20 within module 16. Excess heat from heater 26 is intended to pass through the walls 28 of tank module 16 and locally liquefy solution 20 in proximity to tank module 16 and inlet 22 to allow gravitational replenishment of solution into tank module 16. As described above, a problem exists in operation of prior art systems 10 in that walls 28 of reservoir tank module 16 are typically formed of a plastic polymer having relatively low thermal conductivity such that at extreme low temperature conditions insufficient heat is passed into storage tank 18 to maintain an adequate rate of liquefaction and replenishment.

In a first method for replenishing the reservoir tank module in accordance with the present invention, (and referring now to FIG. 2 and a first embodiment 110 of an improved system for supplying a solution of urea to a diesel exhaust emissions control system) a portion 130 of the exhaust stream 132 from engine 14 is diverted, preferably following the diesel particulate filter 134 and before the engine muffler 136 and tail pipe 138. Portion 130 is preferably diverted by a porting mechanization 140 attached to the vehicle exhaust pipe, such as an electrically-operated valve assembly. Downstream of valve assembly 140, an optional filter 142 may be provided to purify exhaust gas 130 for additional cleanness of the gas into storage tank 118. A delta pressure sensor 144 may be employed across filter 142 to monitor filter condition. Another pressure sensor 146 combined with a temperature sensor 148 on the bypass flow line 150 may be employed to enable calculation of the flow rate of exhaust portion 130. The diverted exhaust gas is directed either into or against the outside of urea storage tank 118. A currently preferred arrangement is to direct the exhaust gas 130 onto or through solution 20 within storage tank 118, whether frozen or liquid, via a sparger 152, in which arrangement cooled exhaust gas 130′ then exits storage tank 118 via a tank vent port 154.

A side benefit of first embodiment 110 is that tank vent 154 is readily thawed in the event that it has been frozen shut by internal splash of urea solution or external road spray. This arrangement eliminates a potential failure mode wherein liquid solution cannot not be pumped from reservoir tank module 16 into diesel exhaust emissions control system 12 (FIG. 1) because built up vacuum in storage tank 118 cannot be relieved via a frozen vent 154.

A second side benefit of first embodiment 110 is that the top surface of frozen urea solution 20 is heated directly by the impinging gasses. This liquid urea solution will descend to the bottom of the storage tank 118 along the walls of the tank module 16 and become available to replenish the tank module 16. This function eliminates the condition where an air gap forms around the perimeter of the tank module 16 after the melted urea solution is drawn away from the surroundings of the tank module 16. This condition would cause the emission abatement system to stop functioning due to lack of supply of liquid urea solution.

This arrangement, whether utilizing hot exhaust gas within storage tank 118 as shown in FIG. 2 or impinging hot exhaust gas against the outside of storage tank 118 (not shown), can create an exhaust-disposal problem if, for example, system 110 is conveniently located within the engine compartment of a vehicle. A second exhaust pipe (not shown) may be required to dispose safely of cooled exhaust gas 130′ from vent 154.

Referring now to FIG. 3, a second embodiment 210 of an improved system for supplying a solution of urea to a diesel exhaust emissions control system avoids this problem. Second embodiment 210 is very similar to first embodiment 110 and employs numerous identical components, so indicated by the same numbers as in FIG. 2. The difference is that, instead of piping exhaust gas per se into or against storage tank 118 as in first embodiment 110, heat is abstracted from the engine exhaust system or a component thereof 224 in known fashion, such as by a conventional finned heat exchanger or other hot air collector 226. (Collector 226 may be placed at any point on the engine exhaust system from the exhaust manifold to the tail pipe, location depending upon the amount of heat desired and the packaging constraints of the installation.) Hot air 230 rather than hot exhaust gas 130 is then fed into system 210 via a line fan 228. The flow path is substantially the same as for hot exhaust gas 130 in FIG. 2; however, a substantial benefit of the slightly greater system complexity (heat collector and fan plus controls) is that only cooled air 230′ exits vent 154.

While the invention has been described by reference to a specific embodiment, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiment, but will have full scope defined by the language of is the following claims. 

1. A system for keeping a solution of urea in a liquid state at normally sub-freezing temperatures or liquefying such solution if frozen, for delivery to an emissions abatement system of an internal combustion engine, comprising: a) a storage tank for said urea solution; b) first apparatus for deriving a hot gas from an exhaust system of said engine; and c) second apparatus for impinging said hot gas against an area of said storage tank to heat said solution of urea therewithin.
 2. A system in accordance with claim 1 wherein said second apparatus includes said storage tank, a gas sparger disposed within said storage tank for dispersing said hot gas, and a tank vent for escape of cooled gas from said tank.
 3. A system in accordance with claim 1 wherein said second apparatus includes at least one of a filter for filtering said hot gas, a temperature sensor, and a pressure sensor.
 4. A system in accordance with claim 1 wherein said first apparatus includes a diverter valve mounted on said exhaust system for diverting a portion of an exhaust gas stream flowing therewithin, and wherein said hot gas is said exhaust gas portion.
 5. A system in accordance with claim 1 wherein said first apparatus includes a hot air collector disposed adjacent said exhaust system.
 6. A system in accordance with claim 5 wherein said first apparatus further includes a fan configured for passing said hot gas into said second apparatus.
 7. A system in accordance with claim 1 wherein said impinged area of said storage tank is selected from the group consisting of within said storage tank and an outer surface of said storage tank.
 8. An internal combustion engine comprising a system for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures, or liquefying such solution if frozen, for delivery to an emissions abatement system of an internal combustion engine, wherein said system includes, a storage tank for said urea solution, first apparatus for deriving a hot gas from an exhaust system of said engine, and second apparatus for impinging said hot gas against an area of said storage tank to heat said solution of urea therewithin.
 9. A method for keeping a reservoir solution of urea in a liquid state at normally sub-freezing temperatures, or liquefying such solution if frozen, for delivery to an emissions abatement system of an internal combustion engine, comprising the steps of: a) providing a storage tank for said urea solution; b) deriving a hot gas from an exhaust system of said engine; and c) impinging said hot gas against an area of said storage tank to heat said solution of urea therewithin.
 10. A method in accordance with claim 9 wherein said hot gas is at least a portion of an exhaust gas stream of said internal combustion engine.
 11. A method in accordance with claim 9 wherein said hot gas is heated air.
 12. A method in accordance with claim 9 wherein said impinging step is selected from the group consisting of impinging said hot gas within said storage tank and impinging said hot gas against an outer surface of said storage tank. 